1. Field of the Invention
The invention relates to a direct injection engine structured such as to directly inject a fuel to a combustion chamber so as to supply, and more particularly to a direct injection engine mainly aiming to reduce a discharging amount of a harmful NOx component as much as possible.
2. Description of the Related Art
Recently, direct injection engines, in which a fuel is directly injected and supplied to combustion chambers, have spread as an on-vehicle four cycle gasoline engine. With such direct injection engines, an intake air is ordinarily sucked to generate a swirling vortex in combustion chambers (swirling, tumble flow or the like) so as to effect stratified charge combustion (combustion in a state, in which a layer of mixture having a rich air-fuel ratio is formed near ignition plugs provided facing combustion chambers, and a layer of mixture having a lean air-fuel ratio (an air layer) is formed toward side wall surfaces (outer peripheries) of the combustion chambers with distance from the ignition plugs. Thereby, super-lean burn operation is made possible, and pumping loss and heat loss are reduced to achieve improvement in fuel consumption.
Further, the invention described in Japanese Unexamined Patent Publication No. 7-103078 relates to an EGR (exhaust gas recirculation) system for direct injection type engines, in which an EGR gas recirculated around an ignition plug facing a central portion of a combustion chamber is concentrated and supplied to the combustion chamber in each of cylinders, and an EGR control valve is provided in an EGR duct to control an amount of EGR gas recirculated.
In the former direct injection engine mentioned above, however, a NOx component discharged during the lean burn operation cannot be purified by a normal ternary catalyst, and is discharged to the atmosphere. Further, since restrictions are put on time of lean burn operation and an amount of NOx in spite of the use of NOx catalysts, reduction in an amount of NOx discharged (generated) in an engine out (combustion chamber) has counted for much. To cope with this, it may be effective to perform EGR for recirculating exhaust gases to combustion chambers via an intake system from an exhaust system.
However, with conventional direct injection engines, execution of EGR enables reduction in an amount of Nox discharged to some extent but deteriorates quality of combustion, which puts restrictions on an amount of exhaust gases being subjected to EGR to make it difficult to perform EGR in a large amount and to make enlargement of stable burn limit impossible.
Also, with the EGR system for direct injection type engines, an EGR gas is supplied around an ignition plug at a central portion, and the EGR gas and an intake air are supplied to a combustion chamber in a swirling manner at the same flow velocity. Therefore, the intake air does not act to positively surround the EGR gas at a centeral portion of the combustion chamber, and stability of stratified charge combustion in a vertical direction of the combustion chamber is not taken account of, resulting in problems that reduction in an amount of NOx discharged should be improved, and when an amount of EGR gas recirculated is increased, reduction in output is caused.
The invention has been made in view of the problems mentioned above, and has its object to provide a direct injection engine capable of reducing an amount of NOx discharged as much as possible, realizing EGR in a large amount, and enlarging a stable burning limit.
In order to achieve the object mentioned above, the invention provides a direct injection engine comprising intake ports for supplying an air and an EGR gas to combustion chambers, fuel injection valves directly injecting a fuel, and ignition plugs, and wherein the air and the EGR gas are supplied to the combustion chambers such that the air having a large flow velocity and the EGR gas having a small flow velocity generate a tumble gas flow so as to provide a state of distribution, in which near the ignition plugs, an air-fuel ratio is on a rich side with a less fresh air and a much EGR gas, and with distance from the ignition plugs, the air-fuel ratio comes to a lean side with a much fresh air and a less EGR gas.
Further, in a preferred embodiment of the invention, the direct injection engine comprises intake ports and combustion chambers, which have fuel injection valves and into which fuel is directly injected, and the intake ports, respectively, are partitioned into a high velocity port portion having a high intake flow velocity and a low velocity port portion having a low intake flow velocity, and an EGR gas is introduced into the low velocity port portions.
Further, in another preferred embodiment of the invention, the direct injection engine comprises combustion chambers, in which fuel injection valves directly injecting a fuel and ignition plugs are arranged, and intake ports adapted to generate a tumble gas flow composed of an air and an EGR gas in the combustion chambers, and partitions compartment the respective intake ports into a high velocity port portion having a high intake flow velocity and a low velocity port portion having a low intake flow velocity, and wherein passage valves for opening and closing the port portions are arranged near upstream ends of the low velocity port portions, and EGR passages for introduction of the EGR gas are connected to the low velocity port portions. Further, in a further preferred embodiment of the invention, it is preferable that the intake ports, respectively, in all the cylinders of the engine are compartmented by partitions into a high velocity port portion having a high intake flow velocity and a low velocity port portion having a low intake flow velocity, and EGR gas introduction ports are provided in the low velocity port portions in all the cylinders.
In another preferred embodiment of the invention, EGR passages are provided to connect the low velocity port portions of the respective cylinders to exhaust passages, make a single common passage portion at upstream sides thereof and make at downstream sides thereof branch passage portions, which branch from the common passage portion to be connected to the respective low velocity port portions, and an EGR control valve is provided in the common passage portion. In a further preferred embodiment of the invention, an amount of the EGR gas introduced is regulated by controlling opening degrees of the passage valves.
Meanwhile, the invention provides a method of effecting stratified charge combustion in a direct injection engine, in which an air and an EGR gas are supplied to combustion chambers and a fuel is directly injected into the combustion chambers, the method comprising the step of effecting stratified charge combustion by producing through a tumble gas flow a state of distribution, in which near ignition plugs in the combustion chambers, an air-fuel ratio is on a rich side with a less fresh air and a much EGR gas, and with distance from the ignition plugs, the air-fuel ratio comes to a lean side with a much fresh air and a less EGR gas. And at the time of initial combustion, combustion is effected in a state, in which combustion temperature is suppressed, and after the middle period of combustion, combustion proceeds in a state, in which the combustion temperature is suppressed.
In a preferred embodiment of a direct injection engine according to the invention constructed in a manner mentioned above, an EGR gas is introduced individually into (low velocity port portions) respective cylinders. Therefore, as compared with a conventional direct injection engine, in which an EGR gas is introduced in lump into a collecting passage portion (in an upstream side from intake ports) of an intake passage, improvement according to the invention is achieved in delivering quality to reduce dispersion in amounts of an EGR gas among respective cylinders, whereby a lean limit air-fuel ratio can be coordinated among all the cylinders to realize EGR in large amount.
Also, in a direct injection engine, an intake air is ordinarily sucked into combustion chambers in a manner as to generate a tumble flow and stratified charge combustion is effected such that an air-fuel ratio becomes rich near ignition plugs, so that when an EGR gas is introduced into the low velocity port portions of the intake ports in the above-mentioned manner, a flow velocity of a swirling vortex flow of an intake air, which contains the EGR gas sucked into the combustion chambers from the low velocity port portions, is smaller than a flow velocity of a swirling vortex of an intake air composed only of an air sucked into the combustion chambers from the high velocity port portions. Accordingly, there are successively formed from near the ignition plugs toward an outer periphery thereof in a vertical direction a layer substantially composed only of the EGR gas, an intake air layer containing EGR gas in a large amount, and an intake air layer substantially composed only of an air, resulting in a state, in which the EGR gas is surrounded by an air (fresh air O2) in a vertical direction. In other words, near the ignition plugs in the combustion chambers, an air-fuel ratio is on a rich side with a little fresh air and a much EGR gas, and comes to a lean side with distance from the ignition plugs to lead to a state of distribution with a much fresh air and a less EGR gas. As a result, an initial combustion occurring near the ignition plugs becomes combustion with an air containing an EGR gas in a large amount, so that as compared with the prior art, combustion decreases in temperature to lead to reduction in an amount of NOx discharged, and a fresh air is supplied during flame propagation after the middle period of combustion, resulting in extinguishment of burning and accidental fire.
In this manner, with the direct injection engine according to the invention, an EGR gas is introduced individually into the respective cylinders and an intake air is stratified in the EGR gas level, so that an amount of an EGR gas is reduced in dispersion among the cylinders as compared with the conventional one, and temperature of burning is decreased. Moreover, since combustion occurs in a state, in which combustion temperature is suppressed, it is possible to sharply reduce an amount of NOx discharged and to enlarge a limit of stable combustion, and further an intake air is stratified to prolong a period of combustion, thus enabling reduction in an amount of HC discharged owing to afterburning or the like.
Further, negative pressures at the EGR gas introduction ports are changed by controlling the opening degrees of the passage valves arranged near the upstream ends of the low velocity port portions, whereby the passage valves make it possible to adjust amounts of EGR gas introduced. Therefore, with the prior art, the use of a stepping motor or the like is necessary to finely control the opening degree of the EGR control valve provided in the EGR passage for controlling an amount of EGR gas. With the invention, simple opening and closing of the EGR control valve suffices, so that the use of an inexpensive electro-magnetic valve (ON and OFF valve) is made possible to achieve reduction in cost.